gistfile1.txt

  @tailrec                                                                                                                       
  def collapse[A](as:List[A])(f:(A, A) => A):Option[A] = as match {                                                              
    case Nil => None                                                                                                             
    case a :: Nil => Some(a)                                                                                                     
    case as => collapse(pairwise(as, Nil)(f))(f)                                                                                 
  }                                                                                                                              
                                                                                                                                 
  @tailrec                                                                                                                       
  def pairwise[A](as:List[A], sofar:List[A])(f:(A, A) => A):List[A] = {                                                          
    as match {                                                                                                                   
      case a1 :: a2 :: as => pairwise(as, f(a1, a2) :: sofar)(f)                                                                 
      case a :: Nil => a :: sofar                                                                                                
      case Nil => sofar                                                                                                          
    }                                                                                                                            
  }